Various designs of the slab have been offered, as well as methods of joining the slab to the concrete enclosure. However, these designs are relatively costly and complex and make use of large masses of steel and numerous welds. The connections with the reactor concrete enclosure are fragile and difficult to make and moreover do not adapt well to the temperature variations occurring during the operation of the reactor. The earthquake resistance of such slabs, in particular their connection to the concrete enclosure, is also relatively poor, which restricts their use at some sites.